CAN Interface

The CAN interface is acheived using an NXP TJA1048, a dual-channel CAN transceiver. This is a part recommended by Ford engineers for it’s reliablity and robustness. The only hard requirement of this transceiver is that it have a standby mode, and some way to wake up the microcontroller when the CAN bus comes out of standby. Aside from the CAN signals, the TJA1048 also has one standby input per channel - pulling each line low will put it’s respective CAN channel into standby mode. These are routed to P0.19 for CAN Channel 1, and P0.6 for CAN Channel 2.

Some support electronics are required on the vehicle side of the CAN transceiver. D4 and D5 provide additional ESD protection against voltage spikes on the CAN line - each PESD1CAN can provide up to 200W of peak dissipation and ESD protection up to 23kV. R1 and R2 provide termination resistance for the CAN Bus. If the VI is acting as a termiantion point for a bench testing CAN rig, these resistors may need to be connected across the HI and LO wires of each CAN bus. SJ1 and SJ2 are closed by default, enabling this termination. If this termination is not needed, the solder jumpers can be cut (see this issue). Finally, each CAN signal has a 100pF bypass capacitor very close to the OBD connector, to shunt high frequency noise to ground.

The CAN bus doesn’t particularly require a high speed PCB design, since the rate is only 1Mbps at most and 500Kbps in practice. However, an attempt was made to maintain the characteristic 120 ohm differential impedance on the CAN lines (Note: this is a mistake in the design, as the termiation resistance should be left to the vehicle’s bus in normal operation - see issue #21). An attempt was also made to maintain a 45 mil guard band around each CAN bus, to keep high frequency switching transients from coupling onto the CAN bus. The guard band was also applied to the LPC1769 side of the CAN transciever, to keep the same transients out of the converted CAN signals. Since the layout is only 2 layers and very crowded, this wasn’t implemented perfectly, but seems to be within 85% coverage. The combination of these considerations means that the CAN interface should be highly resilient to noise and should limit signal distortion due to reflections and other transmission line phenomena.

All CAN transceiver options we considered: